Not Applicable
Not Applicable
The present invention relates to optical sensors, and in particular to a high-precision, micro-optic absorption spectrometer.
During the past few years, a substantial amount of research has been performed in the field of optical microcavity physics, in order to develop high cavity-Q optical microcavity resonators. In general, resonant cavities that can store and recirculate electromagnetic energy at optical frequencies have many useful applications, including high-precision spectroscopy, signal processing, sensing, and filtering. Many difficulties present themselves when conventional planar technology, i.e. etching, is used in order to fabricate high quality optical resonators, because the surfaces must show deviations of less than about a few nanometers. Optical microsphere resonators, on the other hand, can have quality factors that are several orders of magnitude better than typical surface etched optical micro-resonators, because these microcavities can be shaped by natural surface tension forces during a liquid state fabrication. These microcavities are inexpensive, simple to fabricate, and are compatible with integrated optics.
Optical microcavity resonators have quality factors (Qs) that are higher by several orders of magnitude, as compared to other electromagnetic devices. Measured Qs as large at 1010 have been reported. The highs resonances encountered in these microcavities are due to whispering-gallery-modes (WGM) that are supported within the microcavities.
As a result of their small size and high cavity Q, interest has recently grown in potential applications of microcavities to fields such as electro-optics, microlaser development, measurement science, and spectroscopy. By making use of these high Q values, microspheric cavities have the potential to provide unprecedented performance in numerous applications. For example, these microspheric cavities may be useful in applications that call for ultra-narrow linewidths, long energy decay times, large energy densities, and fine sensing of environmental changes, to cite just a few examples.
In order for the potential of microcavity-based devices to be realized, it is necessary to couple light selectively and efficiently into the microspheres. Since the ultra-high Q values of microcavities are the result of energy that is tightly bound inside the cavity, optical energy must be coupled in and out of the high Q cavities, without negatively affecting the Q. Further, the stable integration of the microcavities with the input and output light coupling media should be achieved. Also, controlling the excitation of resonant modes within these microcavities is necessary for proper device performance, but presents a challenge for conventional waveguides.
Typically, good overall performance is gained by accessing the evanescent field in a waveguide. Also, only waveguide structures provide easy alignment and discrete, clearly defined ports. Because of cavity and waveguide mode leakage into the substrate and into the modes within the fiber cladding, power extraction from the input optical radiation has proved to be inefficient for conventional planar waveguides, however.
U.S. patent application Ser. No. 09/893,854 (identified by Attorney Docket Nos. CSLL-625 and hereby incorporated by reference) discloses a highly efficient and robust mechanism for coupling optical microcavity whispering-gallery modes into integrated optical waveguide chips. SPARROW (Stripline Pedestal Antiresonant Reflecting Waveguides) are used to achieve vertical confinement and substrate isolation through a highly reflective stack of alternating high and low refractive index dielectric layers. Q-values of over 1010, and coupling efficiencies of over 98% have been observed.
SPARROW waveguide chips have the potential to integrate optical microcavities into miniaturized optical sensor systems. Because of their ability to excite resonant modes having unprecedentedly high Q-values in optical microcavities, SPARROW waveguide chips have the potential for greatly increasing the resolution and dynamic range in these sensing applications.
In particular, a significant potential application for microcavity resonator devices is chemical/biological agent sensing. Chemical sensors known in the art include MEMS (microelectromechanical systems) chemical sensors, optical waveguide-based sensors, surface plasmon resonance (SPR) chemical sensors, surface acoustic wave (SAW) chemical sensors, mass spectrometers, and IR (infrared) absorption spectrometers Miniaturized sensors, such as prior art MEMS sensors, provide significant advantages. For example, they would be well adapted for in situ functioning. Also, they would be small enough to be deployed in large numbers and implemented for remote probing. It is desirable to provide chemical sensors with an improved resolution, while maintaining the compact size of MEMS sensors known in the art.
The present invention is directed to a light absorption spectrometer, formed of a waveguide-coupled optical microcavity resonator. The present invention features the tuning of the optical resonance frequency of the microsphere, to coincide with a selected electronic or vibrational transition frequency, so that the light coupled into the microsphere will experience absorption in the presence of an atomic or molecular substance surrounding the microsphere.
An infrared absorption spectrometer constructed in accordance with the present invention includes at least one optical microcavity, and an optical waveguide for coupling light into a resonant mode of the optical microcavity. The optical waveguide has an input end and an output end. The waveguide is adapted for transmitting optical radiation incident on the input end to the output end.
The light coupled into the optical microcavity is adapted to interact with at least one an atomic or molecular species. The atomic or molecular species may be found in a chemical substance surrounding the microcavity, and may be a fluid, by way of example. The optical microcavity is configured so that the frequency of at least one resonant mode of the optical cavity matches an electronic or vibrational transition frequency of the atomic or molecular species. In this way, optical radiation coupled into the optical microcavity and having a frequency substantially equal to the frequency of the resonant mode is absorbed by the atomic or molecular species.
Because of the high Q value and the correspondingly long optical path length of the optical microcavity, the sensitivity of the infrared absorption spectrometer of the present invention is significantly increased, as compared to the prior art.